Millimeter-wave Field Effect Transistor ("FET") based MMICs are replacing hybrid circuit functions, such as amplification, mixing and switching. However, currently there are few millimeter-wave monolithic FET based Voltage Controlled Oscillators ("VCOs") and no reports of millimeter-wave high electron mobility transistor ("HEMT") based VCOs.
Additionally, there have been only relatively few attempts at monolithic millimeter-wave GaAs VCO designs and even fewer employ active elements (i.e., metal epitaxial semiconductor FETs: "MESFETs") as tuning elements. In one instance, varactor diode tuning elements have been added to a standard 0.25 um gate-length MESFET fabrication sequence; however, the increased process complexity required (i.e., Me V ion implantation) is difficult, especially in order to ensure compatibility with HEMTs. While conventional FET-only grounded-gate oscillator circuit architecture has no compatibility problems, it exhibits a tuning range and circuit Q performance limitation tradeoff primarily because the diminished gain and higher series resistance of the single FET active element as the gate bias approaches pinchoff.
Previously, as alluded to above, FET oscillators consisted of one FET device utilizing a shorted gate. These devices are commonly known as "common gate" oscillators. While such oscillator devices work in a satisfactory manner, the negative resistance for the device is inherently limited in a narrow band and, therefore, must be redesigned to set negative resistance if a different resonator is used with the device. Additionally, since only one device was previously utilized, a large device to increase output power could not be used while still maintaining the negative resistance required for oscillation.
Therefore, there is a need for a voltage controlled oscillator having very broad band negative resistance, a large tuning bandwidth, active feedback to enable use of a large FET for output power, and lower phase noise.